In general, industrially applicable processes for synthesizing methanol, which is an important starting material for petrochemical materials, include use of synthesis gas produced through the gasification of coal or biomass and reforming of natural gas. Such gasification of coal or biomass generally uses a fluidized-bed reactor, and requires a dust collection system for collecting dust as well as removal systems of sulfur compounds or nitrogen oxides contained in raw materials. Therefore, methanol has been produced mostly (70% or more) by a commercial process using synthesis gas obtained from the reforming of natural gas. In Korea, methanol is in demand of about 1.2 million tons/year, is used in synthetic resins, chemical fiber materials, methyl t-butyl ether (MTBE), acetic acid, etc., and costs $250-500/ton variably.
The annual production capacity of methanol reaches 20 million tons all over the world. However, when methanol partially substitutes for gasoline or diesel fuel, it is expected that methanol is increasingly in demand. In addition, methanol as liquid fuel emits a lower amount of harmful nitrogen oxides (NOx) as compared to gasoline or diesel fuel, and thus it is expected that such high eco-friendly characteristics of methanol results in industrial demand as a fuel substitute. Currently, high cost of methanol limits its use as a fuel substitute. However, due to the recent tendency of high oil price, commercial use of methanol as a non-petroleum fuel substitute has been spotlighted.
Methanol is produced from synthesis gas via the hydrogenation of carbon monoxide or carbon dioxide as depicted in the following reaction formulae:CO+2H2CH3OHΔH=−90.8 kJ/mol  (1)CO2+3H2CH3OH+H2OΔH=−49.6 kJ/mol  (2)CO+H2OCO2+H2ΔH=−41.2 kJ/mol  (3)
Reaction formulae (1) and (2) are kinds of the exothermic volume-reducing reactions, and thus they prefer a low temperature and a high pressure thermodynamically. However, since reaction rate increases in proportion to temperature, industrial production of methanol has been conducted at an adequate temperature. Due to this, an actual commercialized process is operated under a one-pass conversion of reaction gases of 15-25% in order to prevent accumulation of reaction heat. Because of such a low one-pass conversion of methanol, the production cost increases. Meanwhile, the unreacted gases are recirculated and this results in an additional need for a system for synthesizing methanol. If a reaction is carried out according to reaction formula (2) to produce water, a water gas shift reaction (WGS), such as one as shown in reaction formula (3), occurs as a side reaction, thereby forming surplus hydrogen and increasing reaction rate of methanol synthesis.
When using synthesis gas, the types of chemicals produced by the reaction vary significantly depending on the particular reaction process and catalyst type. For example, in the case of the Sasol process (South Africa), using an iron-based catalyst in a fluidized-bed reactor results in production of gasoline (Fuel. Pro. Tech. 48 (1996) 189), while using a Cu/Zn/Al catalyst in a fixed-bed reactor results in facile production of methanol as an important starting chemical. To perform such reactions, as a methanol synthesis catalyst for high-pressure application, BASF Corporation (Germany) has developed a zinc chromate-based catalyst. As a catalyst for low-pressure application, a Cu—based catalyst has been developed. However, such catalysts are susceptible to poisoning with 1 ppm of sulfur compounds. Since an improved catalyst for methanol synthesis has been disclosed by ICI Co. in 1996, various companies have developed high-quality catalysts. Some industrially applicable catalysts for methanol synthesis are summarized in Table 1.
TABLE 1Chemical composition of catalysts for methanol synthesis according torelated artProduction Co.Catalyst systemCompositionAmmonia CasaleCu—Zn—Al—Cr29:47:6:18BASFCu—Zn—Al32:42:26Cu—Zn—Al—Cr—Mn38:38:0.4:12:12DupontCu—Zn—Al50:19:31Halder TopsoeCu—Zn—Cr37:15:48ICICu—Zn—Al61:30:9Cu—Zn—Al64:23:13LurgiCu—Zn—V61:30:9Cu—Mn—V48:30:22Mitsubishi GasCu—Zn-Mp55:43:2ChemicalCu—Zn—Cr55:43:2Cu—Zn—B61:38:1ShellCu—Zn—Ag61:24:15Cu—Zn—RE71:24:5United CatalysisCu—Zn—Al62:21:7
The catalysts developed for low-pressure application use Cu and Zn as main catalyst components and Al or Cr as a co-catalyst component. Currently, a catalyst available from ICI and having a molar ratio of Cu/Zn/Al of 60/30/10 has been used widely. In 1960's, ICI has developed a process for synthesizing methanol from synthesis gas obtained from coal on a ternary catalyst (Cu/Zn/Al2O3) at a reaction temperature of 230-280° C. under a pressure of 50-100 atm. Particularly, a currently industrialized process for synthesizing methanol is operated on a Cu/Zn/Al2O3 catalyst at a reaction temperature of 250° C. under a pressure of 50-100 atm by using synthesis gas (CO/CO2/H2) obtained by steam reforming of natural gas.
In addition to the above, other catalysts for methanol preparation are also reported, and particular examples thereof include: catalysts obtained by co-precipitation of metals in slurry (U.S. Pat. No. 5,221,652; EP 07421193 A1); methanol preparation catalyst systems containing components, such as Cu—Zn—Zr (U.S. Pat. No. 6,504,497), Cu—Zn—Al—Ga (Japanese Laid-Open Patent No. 2002-60357) or Cu—Zn—Al—Zr—Mo (U.S. Pat. No. 5,254,520) modified from the known Cu—Zn—Al; or the like. Further, U.S. Pat. No. 6,342,538 discloses a Pd/CeO2 catalyst system free from Cu and Zn and using ceria with a particle size of 5 nm or less as a carrier.
The synthesis gas suitable for methanol synthesis has a H2/(2CO+3CO2) ratio of about 1.05. Since the yield of methanol increases in proportion to the ratio, it is required to add hydrogen or to remove carbon dioxide to adjust the ratio. Although many workers have participated in studies for improving the performance of methanol synthesis catalysts, understanding about the active sites of a catalyst for methanol synthesis is not completely accomplished. However, it is known that oxidation state of Cu and redox property of reduced Cu particles play an important role in determining the catalytic activity. It is also known that the activity of a Cu catalyst in a reaction of methanol synthesis is in proportion to the specific surface area of metallic Cu component. It is reported that coordination, chemisorption and activation of CO and homogeneous H2 splitting occur on Cu0 or Cu+, and non-homogeneous H2 splitting, leading to Hδ+ and Hδ− in a catalytic system using a ZnO— containing catalyst, occurs on ZnO (Appl. Catal. A 25, (1986) 101). Herein, it is reported that when the molar ratio of Cu/Zn is 8 or more, the specific surface area decreases rapidly (Appl. Catal. A 139, (1996) 75). For this reason, Cu is used in combination with Zn to prepare the catalyst, and a molar ratio of Cu/Zn of 3/7 is known to provide the highest activity. However, it is known that when CO2 is present or when the proportion of oxygen-containing materials that cover the Cu0 surface increases, the catalyst activity is independent from the Cu0 surface area. It is reported that such a phenomenon results from the fact that the Cu+ active site functions as an active site during the methanol synthesis.
To obtain a catalyst for methanol synthesis, ZnO, ZrO2, Cr2O3 and SiO2 are used as carriers or promoters. In the case of ZnO, Zn+ ions are arranged at the site of tetrahedron by O2− ions, and ZnO serves to optimize dispersion of Cu particles in a Cu—based catalyst and to stabilize the active site. ZnO itself also acts as a catalyst for hydrogenation.
When using a catalyst containing a simple mixture of Cu/SiO2 and ZnO/SiO2, ZnO does not serve to deform the shape of Cu sites but contribute to formation of Cu—Zn active sites. In this context, ZnO causes a change in electrical properties of Cu sites by the interaction with Cu particles and electron exchange.
Considering that high-petroleum price is to be maintained continuously in the future, it is expected that there is a rapid increase in utility of methanol as a fuel substitute or reactant for fuel cells. Under these circumstances, it is quite necessary to develop a catalyst system for carrying out an efficient reaction by which methanol is produced from synthesis gas in a cost-efficient manner.
In addition, there have been problems in the related art that generation of carbon dioxide caused by oxidation of carbon monoxide occurring as one of the side reactions on the catalysts for methanol synthesis, as well as generation of hydrocarbons and DME leads to a drop in yield of methanol.